This invention generally relates to the field of paper making, and more specifically, to a fibrous web produced by a transfer system.
In a paper making machine, paper stock is fed onto traveling endless belts or xe2x80x9cfabricsxe2x80x9d that are supported and driven by rolls. These fabrics serve as the papermaking surface of the machine. In many paper making machines, at least two types of fabrics are used: one or more xe2x80x9cformingxe2x80x9d fabrics that receive wet paper stock from a headbox or headboxes, and a xe2x80x9cdryerxe2x80x9d fabric that receives the web from the forming fabric and moves the web through one or more drying stations, which may be through dryers, can dryers, capillary dewatering dryers or the like. In some machines, a separate transfer fabric may be used to carry the newly formed paper web from the forming fabric to the dryer fabric.
Generally speaking, the term xe2x80x9cfirst transferxe2x80x9d refers to the transfer of the wet paper stock from a headbox to the forming fabric, which will be referred to as the xe2x80x9cfirst carrier fabricxe2x80x9d. The term xe2x80x9csecond transferxe2x80x9d may be understood as the transfer of the paper web that is formed on the first carrier fabric to a transfer fabric or a dryer fabric, which will be referred to as a xe2x80x9csecond carrier fabricxe2x80x9d. These terms may be used in connection with twin wire forming machines, Fourdrinier machines and the like.
At or near the second transfer, the first carrier fabric and the second carrier fabric are guided to converge so that the paper web is positioned between the two fabrics. Generally speaking, centripetal acceleration, centrifugal acceleration and/or air pressure (which is typically applied as either a positive pressure or a negative pressure from a xe2x80x9ctransfer headxe2x80x9d that is adjacent to the fabrics) causes the web to separate from the forming fabric and attach to the dryer fabric.
While the second carrier fabric is often run at the same speed as the first carrier fabric, it is known that the second carrier fabric may be run at a speed that is less than the speed of the first carrier fabric. This difference in speed between the fabrics is typically expressed in terms of a ratio of fabric velocities (i.e., velocity ratio) to describe what is known in the industry as xe2x80x9cnegative draw.xe2x80x9d As described in U.S. Pat. No. 4,440,597, to Wells et al., the speed differential between the fabrics in the region of the second transfer bunches the web and creates microfolds that enhance the web""s bulk and absorbency. This increases the bulk and absorbency of the web, and also increases stretch or extensibility in the machine direction (MD) of the web. Too much negative draw, however, will create undesirable xe2x80x9cmacrofoldingxe2x80x9d in which part of the web buckles and folds back on itself. FIG. 1 depicts a cross-sectional representation (not to scale) of an exemplary macrofold in a paper sheet. Generally speaking, macrofolds occur in such a manner that adjacent machine direction spaced portions of the web become stacked on each other in the Z-direction of the web. The risk of macrofolding appears to impose a limitation on the amount of negative draw (i.e., the velocity ratio) that can be applied at the second transfer.
Generally speaking, it has been thought that the amount of MD foreshortening and subsequent extensibility (i.e., MD stretch) imparted to the web at the second transfer is very closely proportional to or essentially the same as the velocity ratio of the second carrier fabric to that of the first carrier fabric. Thus, attempts to increase the MD stretch or foreshortening of a web by increasing the velocity ratio (i.e., negative draw) were thought to also increase the likelihood of macrofolding.
Accordingly, a need exists for an improved process of making a fibrous web with desirable machine direction stretchability while avoiding macrofolding. For example, such a need extends to a process of making a paper web with desirable machine direction stretch while avoiding macrofolding.
There is also a need for an improved second transfer system for use in a paper making machine that allows greater MD extensibility (i.e., MD stretch) to be achieved at the same, or even lower, levels of negative draw than heretofore thought possible. Meeting this need is important because it is highly desirable to achieve greater MD extensibility (i.e., MD stretch) at the same, or even lower, levels of negative draw. It is also highly desirable to achieve even the same amount of MD extensibility (i.e., MD stretch) at lower levels of negative draw. Meeting this need would provide the positive benefits of creating MD-oriented extensibility or stretch in the web while avoiding or lowering the risk of macrofolding. Meeting this need could also allow more MD-oriented extensibility or stretch to be built into the web without increasing the risk of macrofolding.
Furthermore, webs produced by a conventional transfer process using a convex transfer head surface, for example the process described in U.S. Pat. No. 4,440,597, and issued Apr. 3, 1984, may lack sufficient toughness, particularly when wet. Generally, a towel incorporating a web produced by a transfer process with improved toughness provides more durability during scrubbing. In addition, a transfer process produced web with improved toughness may resist deformation and breaking during processing, thereby improving manufacturing efficiencies. Generally moreover, improved toughness permits manufacture of a towel with less strength, but with comparable toughness of a conventional towel. Generally, lowering the strength requirements permits the manufacture of a towel with a softer feel.
Accordingly, a web that is manufactured by a transfer process and has greater toughness will improve over conventional webs.
As used herein, the term xe2x80x9cnonwoven webxe2x80x9d refers to a web that has a structure of individual fibers or filaments which are interlaid forming a matrix, but not in an identifiable repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes known to those skilled in the art such as, for example, meltblowing, spunbonding, wet-forming and various bonded carded web processes.
As used herein, the term xe2x80x9cspunbonded webxe2x80x9d refers to a web of small diameter fibers and/or filaments which are formed by extruding a molten thermoplastic material as filaments from a plurality of fine, usually circular, capillaries in a spinnerette with the diameter of the extruded filaments then being rapidly reduced, for example, by non-eductive or eductive fluid-drawing or other well known spunbonding mechanisms. The production of spunbonded nonwoven webs is illustrated in patents such as Appel, et al., U.S. Pat. No. 4,340,563.
As used herein, the term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high-velocity gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameters, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high-velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. The meltblown process is well-known and is described in various patents and publications, including NRL Report 4364, xe2x80x9cManufacture of Super-Fine Organic Fibersxe2x80x9d by V. A. Wendt, E. L. Boone, and C. D. Fluharty; NRL Report 5265, xe2x80x9cAn Improved Device for the Formation of Super-Fine Thermoplastic Fibersxe2x80x9d by K. D. Lawrence, R. T. Lukas, and J. A. Young; and U.S. Pat. No. 3,849,241, issued Nov. 19, 1974, to Buntin, et al.
As used herein, the term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 100 microns, for example, having a diameter of from about 0.5 microns to about 50 microns, more specifically microfibers may also have an average diameter of from about 1 micron to about 20 microns. Microfibers having an average diameter of about 3 microns or less are commonly referred to as ultra-fine microfibers. A description of an exemplary process of making ultra-fine microfibers may be found in, for example, U.S. Pat. No. 5,213,881, entitled xe2x80x9cA Nonwoven Web With Improved Barrier Propertiesxe2x80x9d.
As used herein, the term xe2x80x9cfibrous cellulosic materialxe2x80x9d refers to a nonwoven web including cellulosic fibers (e.g., pulp) that has a structure of individual fibers which are interlaid, but not in an identifiable repeating manner. Such webs have been, in the past, formed by a variety of nonwoven manufacturing processes known to those skilled in the art such as, for example, air-forming, wet-forming and/or paper-making processes. Exemplary fibrous cellulosic materials include papers, tissues and the like. Such materials can be treated to impart desired properties utilizing processes such as, for example, calendering, creping, hydraulic needling, hydraulic entangling and the like. Generally speaking, the fibrous cellulosic material may be prepared from cellulose fibers from synthetic sources or sources such as woody and non-woody plants. Woody plants include, for example, deciduous and coniferous trees. Non-woody plants include, for example, cotton, flax, esparto grass, milkweed, straw, jute, hemp, and bagasse. The cellulose fibers may be modified by various treatments such as, for example, thermal, chemical and/or mechanical treatments. It is contemplated that reconstituted and/or synthetic cellulose fibers may be used and/or blended with other cellulose fibers of the fibrous cellulosic material. Fibrous cellulosic materials may also be composite materials containing cellulosic fibers and one or more non-cellulosic fibers and/or filaments. A description of a fibrous cellulosic composite material may be found in, for example, U.S. Pat. No. 5,284,703.
As used herein, the term xe2x80x9cpulpxe2x80x9d refers to cellulosic fibrous material from sources such as woody and non-woody plants. Woody plants include, for example, deciduous and coniferous trees. Non-woody plants include, for example, cotton, flax, esparto grass, milkweed, straw, jute, hemp, and bagasse. Pulp may be modified by various treatments such as, for example, thermal, chemical and/or mechanical treatments.
As used herein, the term xe2x80x9cmachine directionxe2x80x9d (hereinafter may be referred to as xe2x80x9cMDxe2x80x9d) is the direction of a material parallel to its forward direction during processing.
As used herein, the term xe2x80x9ccross directionxe2x80x9d (hereinafter may be referred to as xe2x80x9cCDxe2x80x9d) is the direction of a material perpendicular to its machine direction.
As used herein, the term xe2x80x9cmachine direction tensilexe2x80x9d (hereinafter may be referred to as xe2x80x9cMDTxe2x80x9d) is the force per machine direction unit width required to rupture a sample and may be reported as kilogram-force per meter.
As used herein, the term xe2x80x9ccross direction tensilexe2x80x9d (hereinafter may be referred to as xe2x80x9cCDTxe2x80x9d) is the force per cross direction unit width required to rupture a sample and may be reported as kilogram-force per meter.
As used herein, the term xe2x80x9cbasis weightxe2x80x9d (hereinafter may be referred to as xe2x80x9cBWxe2x80x9d) is the weight per unit area of a sample and may be reported as kilogram-force per meter squared.
As used herein, the term xe2x80x9cgeometric mean breaking lengthxe2x80x9d (hereinafter may be referred to as xe2x80x9cGMBLxe2x80x9d) is the measurement of the strength of a material, generally a fabric or nonwoven web, and may be reported in length measurements, such as meters. The greater the geometric mean breaking length generally relates to a stronger material. The geometric mean breaking length is calculated by the formula:
GMBL=(MDT*CDT)0.5/BW
As used herein, the term xe2x80x9cpeak energyxe2x80x9d is the measurement the toughness of a material, generally a fabric or nonwoven web, and may be reported in static energy measurements, such as kilogram times meter times centimeter, which may be hereinafter be abbreviated as xe2x80x9ccm-kgmxe2x80x9d. The peak energy is the area under the tensile load versus strain curve from the origin to the breaking point of the material.
As used herein, the term xe2x80x9cwet mullen burstxe2x80x9d is a test used to measure the overall toughness of a water saturated material, such as fabric or nonwoven web. The higher material rupture pressure, typically reported in pascals, generally relates to a tougher water saturated material.
As used herein, the term xe2x80x9cdry mullen burstxe2x80x9d is a test used to measure the overall toughness of a material, such as fabric or nonwoven web, treated approximately 12 hours at 23 degrees centigrade at 50 percent humidity prior to testing. The higher material rupture pressure, typically reported in pascals, generally relates to a tougher material.
As used herein, the term xe2x80x9cgauge lengthxe2x80x9d is the length of a sample, typically reported in centimeters, measured between the points of attachment while under uniform tension.
As used herein, the term xe2x80x9cslackxe2x80x9d is the lack of tension in a sample and reported in length measurements, such as millimeters.
As used herein, the term xe2x80x9cpercent stretchxe2x80x9d is a test used to measure the toughness of a material, such as fabric or nonwoven web. The percent stretch is the increase in length expressed as a percentage of the corrected gauge length, which is gauge length plus slack. The higher percent stretch generally relates to a tougher material.
As used herein, the term xe2x80x9celmendorf tearxe2x80x9d is a test used to measure the toughness of a material, such as fabric or nonwoven web. The test measures the force, typically reported in centinewtons, required to start or propagate a rip in a material. The higher required force generally relates to a tougher material.
As used herein, the term xe2x80x9ctensile modulusxe2x80x9d is the slope of the tensile load versus strain curve measured from the origin until the sample reaches its inelastic point. This measurement may be reported in units of force per area, such as gram-force per centimeter squared. The higher curve slope generally relates to a tougher sample.
As used herein, the term xe2x80x9ccalenderxe2x80x9d refers to a process for fabrics or nonwoven webs that reduces the caliper and imparts surface effects, such as increased gloss and smoothness. Generally, the process includes passing the fabric through two or more heavy rollers, sometimes heated, and under heavy pressure.
As used herein, the term xe2x80x9cnoncalenderxe2x80x9d refers to a fabric or nonwoven web that has not undergone a calender process.
As used herein, the terms xe2x80x9cpermeablexe2x80x9d and xe2x80x9cpermeabilityxe2x80x9d refer to the ability of a fluid, such as, for example, a gas to pass through a particular porous material. Permeability may be expressed in units of volume per unit time per unit area, for example, (cubic feet per minute) per square foot of material (e.g., (ft3/minute/ft2). Permeability was determined utilizing a Frazier Air Permeability Tester available from the Frazier Precision Instrument Company and measured in accordance with Federal Test Method 5450, Standard No. 191A, except that the sample size was 8xe2x80x3xc3x978xe2x80x3 instead of 7xe2x80x3xc3x977xe2x80x3. Although permeability is generally expressed as the ability of air or other gas to pass through a permeable sheet, sufficient levels of gas permeability may correspond to levels of liquid permeability to enable the practice of the present invention. For example, a sufficient level of gas permeability may allow an adequate level of liquid to pass through a permeable sheet with or without assistance of a driving force such as, for example, an applied vacuum or applied gas pressure.
Accordingly, it is an object of this invention to provide an improved process of making a fibrous web with desirable machine direction stretch while avoiding macrofolding.
It is also an object of this invention to provide a second transfer system for use in a paper making machine that allows greater machine direction stretch to be achieved at the same, or even lower, levels of negative draw than heretofore thought possible.
It is also an object of this invention to provide a fibrous cellulosic web having a relatively low density structure, good absorbency, good strength and relatively high levels of MD extensibility or stretch than heretofore thought possible without macrofolding.
These and other objects are addressed by the process of the present invention for making a machine direction extensible fibrous web utilizing an improved second transfer system having a lengthened transfer zone. The process includes the steps of: 1) forming a fibrous web from an liquid suspension of fibrous material, the fibrous web having a consistency ranging from about 12% to about 38% (after the headbox); 2) transporting the fibrous web on a first carrier fabric at a first velocity to a lengthened transfer zone that begins at a transfer shoe and terminates at a portion of a transfer head and has a machine direction oriented length ranging from about 0.75 inches to about 10 inches; 3) guiding the first carrier fabric and fibrous web over the transfer shoe so they converge at a first angle with a second carrier fabric moving along a linear path through the lengthened transfer zone at a second velocity which is less than the first velocity, wherein the first angle is sufficient to generate centrifugal force to aid transfer of the fibrous web to a second carrier fabric and wherein the first and second carrier fabrics begin diverging immediately after the transfer shoe at a second angle such that the distance between the first and second carrier fabrics through the lengthened transfer zone is approximately equal to the thickness of the fibrous web; 4) applying a sufficient level of gaseous pressure differential at the transfer head to complete the separation of the fibrous web from the first carrier fabric and attachment to the second carrier fabric; and 5) drying the fibrous web.
The fibrous web (e.g., paper sheets) produced by the process of the present invention has greater machine direction extensibility than fibrous webs (e.g., paper sheets) processed with the same carrier fabrics in differential speed transfer processes without the improved second transfer system having a lengthened transfer zone.
According to the invention, the fibrous web may have a consistency ranging from about 18% to about 30%. For example, the fibrous web may have a consistency ranging from about 20% to about 28%.
The lengthened transfer zone begins at a transfer shoe and terminates at a portion of a transfer head. Desirably, the lengthened transfer zone terminates at a leading or top edge of a vacuum slot in the transfer head. When measured between the transfer shoe land and the leading or top edge of a vacuum slot in the transfer head, the machine direction oriented length of the lengthened transfer zone may range from about 0.75 to about 10 inches. For example, the machine direction oriented length of the lengthened transfer zone may range from about 2 to about 5 inches. As another example, the machine direction oriented length of the lengthened transfer zone may range from about 3 to about 4 inches. As yet another example, the machine direction oriented length of the lengthened transfer zone may be about 3.5 inches. Of course, it is contemplated that the lengthened transfer zone having similar dimensions may terminate at other portions of the transfer head such as, for example, the trailing edge of the vacuum slot, the trailing edge of the transfer head or the like.
The first angle at the transfer shoe may range from about 2 degrees to about 20 degrees. For example, the first angle at the transfer shoe may range from about 8 degrees to about 12 degrees.
According to an aspect of the invention, the first and second carrier fabrics diverge immediately after the transfer shoe at a second angle ranging from about 0.01 degree to about 1 degree such that the distance between the first and second carrier fabrics through the lengthened transfer zone is approximately equal to the thickness of the fibrous web. For example, the second angle may range from about 0.075 degree to about 0.5 degree. As another example, the second angle may be about 0.1 degree. Generally speaking, the distance between the first and second carrier fabrics through the lengthened transfer zone may range from about 0.0075 inch to about 0.0125 inch for a paper sheet having a basis weight of about 32 grams per square meter (xcx9c1 ounce per square yard).
In an embodiment of the process of the present invention, the fibrous web may be a paper sheet including, but not limited to, paper towel, paper tissue, crepe wadding, paper napkin, or the like.
The process of the present invention may utilize any conventional drying technique. Desirably, the drying technique is a non-compressive drying technique. Exemplary drying techniques include, but are not limited to, Yankee dryers, heated cans, through-air dryers, infra-red dryers, heated ovens, microwave dryers and the like. The process of the present invention may also include any conventional post-treatment steps including, but not limited to, creping, double re-recreping, mechanical softening, embossing, printing or the like.
The present invention also encompasses a machine direction extensible fibrous web formed by the process described above.
An aspect of the present invention relates to an improved transfer configuration for a paper making machine that is designed to produce in a fibrous web, at any given amount of negative draw, a greater amount of machine direction-oriented extensibility or stretch than was heretofore thought possible. This improved transfer configuration includes first carrier fabric having a first surface on which a fibrous web is transported to the transfer configuration; a second carrier fabric having a second surface on which the fibrous web is transported away from the transfer configuration; and a lengthened transfer zone structure for constraining the first and second carrier fabrics to move through a substantially linear, lengthened transfer zone, the lengthened transfer zone defined as the area in which the first and second surfaces are separated by a distance that is approximately equal to the thickness of the fibrous web, and wherein the lengthened transfer zone structure further constrains the first and second carrier fabrics as to cause the transfer zone to have a machine direction oriented length that is within the range of about 1.5 inches to about ten inches, the lengthened transfer means having the ability to increase the amount of machine direction stretch or extensibility that is built into the fibrous web at any given level of negative draw.
Generally speaking, the distance between the first and second carrier fabrics within the transfer zone should be sufficient so that both the first carrier fabric and the second carrier fabric are in contact with the fibrous web.
An aspect of the improved transfer configuration of the present invention is that the first and second carrier fabrics are constrained so as to form a substantially linear, lengthened transfer zone. The second carrier fabric should pass through the lengthened transfer zone along a linear path. The first carrier fabric should also pass through the lengthened transfer zone along a linear path. The fabrics may diverge at a slight angle which may range from about 0.05 to about 0.125 degrees.
The present invention also encompasses a process of making a machine direction extensible or stretchable fibrous web in which the process includes the steps of (a) transporting a fibrous web on a first surface of a first carrier fabric to a transfer configuration; (b) moving a second carrier fabric that has a second surface to the transfer configuration, the second carrier fabric being moved at a speed that is less than the speed of the first carrier fabric to create an amount of negative draw; (c) constraining, at the transfer configuration, the first and second carrier fabrics to move through a lengthened transfer zone that is defined as the area in which the first and second surfaces are separated by a distance that is approximately equal to the thickness of the fibrous web, the transfer zone having a machine direction oriented length that is within the range of about 1.5 inches to about ten inches; and (d) transporting the foreshortened web away from the transfer configuration on the second surface of the second carrier fabric.
According to an aspect of the process described above, the distance between the first and second carrier fabrics within the transfer zone should be sufficient so that both the first carrier fabric and the second carrier fabric are in contact with the fibrous web.
A machine direction stretchable web made according to the transfer system or process discussed above is also considered to be an important aspect of the invention.
The present invention further encompasses a machine direction extensible noncalendered fibrous web produced by a transfer system of at least eight percent negative draw including a matrix of fibrous web material having a wet mullen burst pressure at least about 10 percent greater than a convex transfer system produced web. Moreover, the matrix of fibrous web material has a wet mullen burst of at least about 74500 pascals. In addition, the matrix of fibrous web material has a GMBL ranging from about 2047 to about 2704. Furthermore, the fibers of the fibrous web matrix may be generated from the group consisting of a bonded carded web, spunbonded web, meltblown fiber web, and multi-ply fibrous web. Moreover, the matrix of fibrous web material may have an elmendorf tear greater than about 66.5 centinewton. Also, the matrix of fibrous web material may have a tensile modulus of at least about 1544 gram per centimeter squared. Additionally, the matrix of fibrous web material may have greater strength at lower negative draw percent. Furthermore, the matrix of fibrous web material may have a greater machine direction toughness at about the same GMBL as a convex transfer produced web.
The present invention still further encompasses a noncalendered paper sheet produced by a transfer system of at least eight percent negative draw including a matrix of fibrous web material having a wet mullen burst pressure at least about 10 percent greater than a convex transfer system produced sheet. In addition, the sheet may have a wet mullen burst of at least about 74500 pascals. Moreover, the sheet may have a GMBL ranging from about 2047 to about 2704. Furthermore, the matrix of fibrous web material may be made of a mixture of fibers and at least one other fiber selected from the group consisting of wood pulp and staple fibers. Moreover, the matrix of fibrous web material may be made of a mixture of fibers and at least one particulate selected from the group consisting of activated carbon, clays, fillers, adsorbents, zeolites, superabsorbents, silica, and hydrocolloid. Additionally, the matrix of fibrous web material may be selected from the group consisting of a bonded carded web, spunbonded web, meltblown fiber web, and multi-ply fibrous web. Also, the matrix of fibrous web material may have an elmendorf tear greater than about 66.5 centinewton. Furthermore, the matrix of fibrous web material may have a tensile modulus of at least about 1544 gram per centimeter squared. Moreover, the matrix of fibrous web material may have greater strength at lower negative draw percent. Also, the matrix of fibrous web material may have greater machine direction toughness at about the same GMBL as a convex transfer produced web.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.